214 



HABIT AND INTELLIGENCE. 



[chap. 



and in 



species. 



Species of 

 Eqiius. 



Laws of 

 variatioa 



and of 

 reversion. 



That wbicli is true of the varieties of the pigeon is also 

 true of the species of the genus Equus. The zebra and 

 other species of Equus are striped : the ass usually has 

 stripes on the back and shoulders only, though sometimes 

 on the legs also : the horse, in general, has no stripes at 

 all ; but horses are occasionally found with stripes on the 

 back and shoulder, and in the Kattywar breed in India 

 these are usual, and bars on the legs are common. The 

 fact of these stripes being characteristic of other species of 

 the genus, and occasionally appearing in the horse, suggests 

 that their appearance in the horse is a case of reversion ; 

 that all the species of Equus, like aR the varieties of the 

 pigeon, are the descendants of one ancestral species, and 

 that the appearance of these stripes is a case of reversion 

 to its characters. What greatly strengthens this presump- 

 tion is, that mixture of race between different species of 

 Equus, as between different varieties of the pigeon, in- 

 creases the tendency to the production of these characters. 

 Bars on the legs are much more common in the mule 

 between the horse and the ass than in either the horse or 

 the ass.^ 



We thus find this law of variation, that characters which 

 are variable as between the species of a genus are also apt 

 to be variable as between the varieties and between the 

 individuals of a species. And we also find this law of 

 reversion, that individuals are sometimes found which 

 present what appears to be the character of the original 

 form from which all the varieties of a species, or all the 

 species of a genus, have been descended. I believe that 

 these laws are true on a much wider scale,^ and will 

 elucidate relations between species, not only of different 

 genera, but of different classes. 



The law, that those characters which are variable as 

 between the species of a genus are also variable as between 



'^ Darwin's Origia of Species, p. 191. 



2 Darwin thinks that the occasional formation of a sixth finger in man 

 may be due to reversion to the characters of a remote ancestor near the 

 bottom of the vertebrate scale. No living species of aii'-breathing vertebrate 

 has more than five digits, but that number is exceeded in fishes and some 

 extinct reptiles. (Darwin's Variation under Domestication, vol. ii. p. 16.) 



